Method and apparatus for solidifying steel ingots



March 3, 1959 P. J. HUGHES 2,375,483

METHOD AND APPARATUS FOR SOLIDIFYING STEEL INGOTS Filed Aug. 1, 1956 3 Sheets-Sheet l a INVENTOR. 225g PETER J. HUGHES 2 E1: 8 8 ATTORNEYS March 3, 1959' P. J. HUGHES 2,875,483 METHOD AND APPARATUS FOR SOLIDIFYING STEEL mcows Filed Aug. 1, 1956 5 Sheets-Sheet 2 FIG. 2.

.INVENTOR. PETER J. HUGHES ATTOR N EYS 2,875,483 METHOD AND APPARATUS FOR SOLIDIFYING STEEL I NGOTS Filed Aug. 1, 1956 P. J. HUGHES March 3, 1959 s Shefs-Sheet s JNVENTOR. PETER J. HUGHES BY -K-a.- \-h\ ATTORNEYS .Figure l is an United States Patent O METHOD AND APPARATUS FOR SOLIDIFYING STEEL INGOTS Peter J. Hughes, Philadelphia, Pa. Application August 1, 1956, Serial No. 601,440 8 Claims. (Cl. 22-73) This invention relates toa method and apparatus for solidifying ferrous and non-ferrous metal ingots and more particularly to the production of ingots having more uniform crystalline solidification patterns than have been heretofore obtained and in which the formation of dendritic crystalline patterns and the existence of voids resulting from dissolved gases coming out of solution during solidification of the ingot are avoided.

Heretofore in the production of ingots a mass of molten metal is poured into an iron mold wherein it is permitted to cool by conductivity of heat from the molten metal into the iron mold and by radiation and convection cooling of the mold. Such cooling quite naturally occurs from the outside of the mass of metal and results in the exterior portion of the mass of metal solidifying prior to the interior portion thereof. This cooling procedure results in the growth of dendrite crystals from the outer portion of the body of metal toward the inner portion thereof as solidification of the metal progresses. The undesirable properties of dendritic structures are well-known in the art and practices such as fiuting of the inside of the mold have been employed to control growth of the dendrite crystals. 1

Furthermore, the progressive solidification is accompanied by the formation of stresses within the body of metal resulting from shrinking occurring within thebody during solidification and contraction of the outer previously solidified portion of the metal being limited due 'to the solid state of the metal. This condition'of stress frequently gives rise to lthe existence of fracture lines along the crystal boundaries. This shrinkage also results in the formation of a depression in the top of the solidified body and it is common practice to provide a "fhot top above the mold within which the shrinkage occurs. After the ingothas solidified the hot top is cut olf and represents waste. This waste is however less than would occur if a hot top ,werenot employed.

A still further difficulty encountered is that as the metal cools solidified gases dissolved therein migrate into and toward the .central' region of the body of metal until, .as this region solidifies, the gases come out of solution and form voids in the ingot. i

' 'It is the primary object of this invention to produce anringot which does nothave a coarsedendritic structure and which does not have the fractures or voids discussed above. a

It is a further objeetof this invention to provide apparatus by means of which an improved ingot can be obtained.

These and other objects of the invention relating in particular to the method involved therein and the construction of apparatus for carrying out the method will begome evident from the following description when read in conjunction with the aecornpanying drawings in which: elevationof the apparatus partly in section;

Figure 2 isa transverse section through the apparatus shew-n in Figured taken onthe trace Z-Jtherein; and

described.

Figure 3 is a longitudinal section through another form of apparatus for carrying out the invention.

In the drawings there is indicated generally at 10 a mold for receiving molten metal. The metal rests upon a base plate 12 which is supported from a suitable surface 14 by means of rubber mounting blocks 16 positioned between steel plates 18.

Resting upon the base plate 12 is a cup-shaped member 20 of an electrical insulating material such as a ceramic material containing spaced coils 22 for conducting electrical current for induction heating as will be hereinafter The coils 22 are hollow as indicated at 24 for the circulation of a liquid coolant for temperature control. The conductors 22 are brought out of the mold as indicated at 26 and pass through the insulating sheath 28 to an induction heating generator and control 30. Induction heating generators and control systems therefore are well known in the art and any conventional type of generator and controller having suitable capacity may be employed.

The interior of the cup-shaped member 20 is lined with a suitable heat insulating material as indicatedat 32 in order to protect the electrical conductors 24 and the insulating body mounting these conductors from the heat existing within the mold. Inside of the insulating layer 32 is a layer 34 of graphite or high temperature heat resistent alloy such as chrome-nickel steel forming a heating element in which eddy currents are set up by the :field produced around the electrical conductors 22 and in which heat is generated for heating of the mold and the metal contained in the mold which will be hereinafter described.

cooling fluid discharges out of the uppermost ends of the passages 38 at the top of the mold.

The mold is provided with a lid indicated at 42 which fits in sealing relation with the uppermost rim of the inner liner 36. The lid is provided with a port 44 to which is connected a vacuum line 46 through which the interior of the mold may be evacuated by connection to conventional vacuum pumping means. The lid 42 is preferably formedof an inner surface 48 of ceramic heat resistant material and an outer surface 5% of suitable high temperature heat insulating material.

The apparatus described provides a mold which may be preheated by means of the induction heating coils and eddy currents generated in the electrically conductive layer 34 in order to preheat the interior of the mold to a temperature slightly above the freezing temperature of the metal to be solidified into an ingot. With the mold thus preheated the metal to be solidified is poured into the mold in aliquid state to a substantial level therein as indicated at 52 and floating on the surface of the metal will be a blanket of slag or other material conventionally occurring.

After the molten metal has been poured into the mold, the lid of the mold is positioned thereon and the space within the mold above the molten metal therein is evacuated. It will be evident that this evacuation serves to induce the release of dissolved gases from the molten metal not only initially but during the subsequent cooling process as will be hereinafter described.

After the mold is closed and the space at the top of a cooling fluid through the passages 38 this will generally not be employed and during the cooling process in order to maintain a relatively uniform temperature throughout the body of metal in the mold the'induction heating means will be operated at successive intervals in order to prevent the outer portion of the body of metal as indicated in the region 56 from falling to a temperature substantially below the metal in the central region of the body as indicated at 58. Thus by means of a slow cooling process including occasional heating'of the radially outer portion of the body of molten metal contained within the mold the entire body can be cooled slowly and uniformly down to its freezing temperature.

As previously described the mold is positioned upon the plurality of rubber pads 16. These rubber pads may be replaced with any suitable vibration damping support means such as springs, air and the like, the important consideration being that during the cooling process the g .body of metal is held stationary and quiet and thus the crystalline patterns, avoids the concentration of stresses and the formation of fractures resulting from the stresses in the central region of the ingot, and avoids the accumulation of dissolved gases in the central region of the ingot. In connection with the absence of the accumulation of gases, it should be noted that the vacuum condition existing over the top of the body of molten metal serves to induce the liberation of these gases during the entire cooling process and thus upon final solidification of the metal there is not only a condition of uniform disposition of remaining solidified gases but also there is a condition of reduced dissolved gas content from that existing at the time the metal is poured into the mould.

While the apparatus which I have disclosed is particularly desirably employed for carrying out my process it will be evident that the process of cooling a body of metal within an evacuated chamber accompanied by the occasional heating of the outermost portion of the body of metal in order to insure uniformity of temperature throughout the entire body during the cooling process may be carried out by various other arrangements of apparatus.

The essential steps to the method are as follows:

(1) Prcheating a mold which is to receive molten material to be solidified to a temperature above the freezing temperature of the material to be solidified.

(2) Delivering molten material to be solidified to the mold.

(3) Closing the mold and evacuating the space within the mold above the body of liquid material and maintaining the vacuum during subsequent cooling of the material.

(4) Cooling the body of liquid material slowly while occasionally applying heat to prevent the exterior portion of the body from cooling more rapidly than the interior portion of the body and thus maintaining a substantially uniform temperature throughout the entire body during the cooling thereof.

(5) Maintaining the body of liquid material substantially motionless during the uniform cooling thereof to a temperature equal to or slightly below the solidification temperature of the material; and

(6) Thereafter rapidly cooling the body of liquid material to induce a substantially simultaneous solidification of the entire body.

The apparatus described in connection with Figures 1 and 2 is particularly desirably employed for carrying out the foregoing process for the reason that it permits both heating and cooling of the mold either alone or with a body of metal therein. It is particularly noted that in the heating process the heat is not generated in the body of metal 52 but rather the heat is generated in the electrically conductive liner 34 which in turn serves to heat the metal by conduction through the mold 36. The generation of heat in the liner 34 provides not only for the heating of the mold prior to the pouring of metal therein but also avoids eddy current stirring of the metal when heat is being applied to the body of metal. In other words, the conductive liner 34 serves to shield the body of molten metal from any magnetic stirring effects resulting from the induction heating process.

As shown in Figure 1, thermocouples 60 may be positioned in the ceramic inner liner 36 and connected through conventional conductors 62 to a suitable tetnperature indicating device 64. Thus, the temperature at various portions of the ceramic mould 36 may be continuously indicated and thus the temperature of the metal within the mold may be determined to a fair degree of accuracy.

As noted above, it is frequently possible to cool the body of metal to a temperature below its freezing temperature before the formation of solidification crystals occurs. When this is done, it is desirable to provide means for agitating the mold in order to induce the initial formation of crystals. Such apparatus is shown in Figure 1 in the form of an air cylinder 66 connected to the base 14 by means of a pivoted mounting 68 and having its piston rod 70 connected to the face plate 12 of the mold by means of a pivotal connection 72. This atrangement provides, if a supply of compressed air is suddenly delivered to the cylinder, for imposing on the mold a shock or impact serving to induce crystal formation. It will be'evident that if the entire body of metal within the mold is cooled to a temperature below the freezing temperature and crystal growth is initiated by means of an impact, the entire body of metal will solidify substantially simultaneously. This method of solidification provides an ingot having extremely uniform structure and substantially completely absent of dendritic crystal structure. I

The apparatus shown in Figures 1 and 2 is employed for the production of a solid billet. If a hollow billet is to be formed, centrifugal apparatus such as indicated generally at in Figure 3 may be employed.

In Figure 3 there is shown mounted on a suitable base 82 a support structure 84 mounting drive and speed reducing means 86 for driving a shaft 88 alfixed to a mold 90. The shaft 88 is mounted on the base 84 by means of a conventional type of bearing 92 and is afiixed to the base of the mold by means of an attaching flange 94. The mold is an open-ended mold having a lid 96 closing its open end and supported by a shaft 98 mounted for rotation in a bearing 100 supported on mounting means 102 longitudinally slidable on a base plate 104 which is in turn supported by a supporting surface 82. It will be evident when the lid 96 is in place in the top of the mold with the tapered lid and mold surfaces 106 in engagement with each other, the mold mounted, on the shafts 88 and 98 which are supported by bearings 92 and 100, respectively, may be rotated by the driving means 86. When it is desired to open the mold the lid 96 may be moved to the right, as viewed in Figure 3, by movement of the bearing mounting means 102 to the right on the base plate 104.

The mold 90 includes a ceramic inner liner 108 corresponding to the mold liner 36 discussed in connection with Figures 1 and 2. The mold liner is provided with air passages 110 extending from the central region of the base, of the mold where they are connected with a passage r12 extending longitudinally through the shaft 88, tothe upper part of; the mold; where they discharge intothe atmosphere as indicated at 114. A bearing fitting 116 is mounted on the support 84 and is provided with a radially extending bore adapted to receive an elastic fluid supply line 118 in communication with an annular recess 120 which communicates with the bore 112 radially extending bores 122 in the shaft 88. This flow of elastic fluid provides for cooling of the mold in the same manner as the flow of fluid through the passages 38 described in connection with Figures 1 and 2.

Surrounding the outer surface of the mold 108 is an annular band of a conductive material 124. This conductive band may be formed of graphite or high temperature heat resistant alloy metal forming a heating element in which eddy currents are set up as will be hereafter described.

Positioned annularly exteriorly of the conductive layer 124 is a layer of heat insulating material 126 which also extends across the base of the mold as indicated at 128. This insulating layer serves to restrict the rate of cooling of the mold 108 and to restrict the transfer of heat outwardly into the induction heating conductors which will now be described.

Positioned externally of the mold and fixed to the base 82 is an annular band of insulating material 130 carrying hollow electrical conductors 132 which are connected to suitable induction heating and control rneans not shown. The annular band of insulating material 130 is spaced from the outer surface of the mold insulating outer covering 126 in order to provide necessary clearance for rotation of the mold assembly. However, during this rotation, if the conductors 132 are suitably energized, induction heating of the conductive annular band 124 will take place, causing heat to be transmitted through the mold 108 and into the body of metal 134 contained therein. Thus, induction heating of the mold and metal carried within the mold may be accomplished during rotation.

In order to provide for filling of the mold, a plug 136 is provided in the mold cover 96. The plug 136 is removed from the cover and suitable pouring spout means may be inserted therein while the molten metal is poured into the mold. Thereafter, the pouring spout is removed and the plug 136 inserted into position within the cover 96 and rotation of the mold commenced.

The shaft 98 is provided with a longitudinally extending central bore 138 connected by means of a suitable swivel fitting 140 to a vacuum line 142. The bore 138 extends through the mold lid 96 to the interior of the mold. Thus, the interior of the mold may be evacuated before and during rotation thereof and during solidification of the metal 134 contained therein.

Carrying out my method in connection with the apparatus described in Figure 3 involves the following steps:

(1) Preheating the mold by means of the induction heating means to a temperature higher than the freezing temperature of the metal to be solidified.

(2) Delivering molten metal to be solidified to the mold through the mold plug 136 or into the mold body top removed if suitable support means are provided.

(3) Closing the top of the mold, evacuating the interior and commencing rotation of the mold when a sulficient degree of vacuum has been obtained therein. The speed of rotation of the mold is selected to be just suflicient to avoid raining of the metal and to provide a smooth annular layer of the molten metal in the mold.

(4) Cooling the annular body of liquid material slowly while occasionally applying heat to prevent the exterior portion of the body from cooling more rapidly than the interior portion of the body. Thus a substantially uniform temperature is maintained throughout the entire annular body of liquid material during cooling thereof. The speed of rotation of and the shielded induction heating arrangement avoids agitation of the annular layer of liquid metaliand only minor vibrationswill occur therein due to slight-unbalancesin the rotating-apparatus.

(5) Cooling the annularbody of material rapidly after its temperature has been uniformly lowered to the. temperature equal to or slightly below the solidification temperature of the material.

When apparatus such as shownin Figure 3 is employed in carrying out the process, some undercooling, i. e-., cooling below the freezing temperature of the metal, can be obtained. It is, however, unnecessary to provide impact or vibrating means such as is provided by the cylinder 66 described in connection with Figure 1. for the reason that there does exist in the rotating body sufficient vibration to induce the initial formation of crystals before the temperature of the material has reached. a temperaturemuch below the freezing temperature. How; ever, particularly in the: cooling of complex and heat resistant alloys, a substantial amount of undercooling can be obtained and in the casting of these alloys, due to their cost and due to the degree of perfection required therein, my method and apparatus as disclosed herein has particular utility. While the apparatus disclosed herein as is particularly desirably employed for the carrying out of my method is obviously not limited to the forms of apparatus disclosed herein but may be carried out by means of apparatus having various other physical forms, provided the apparatus suitably provides for the carrying out of the necessary steps of the method.

What is claimed is l. The method of solidifying a casting comprising preheating a mold in which a coating is to be formed to a temperature above the freezing temperature of the ma terial forming the casting, depositing into the mold liquid material to be solidified and uniformly cooling the body of liquid material in the mold throughout its cross section and bringing the temperature of the material throughout the body'to its freezing temperature at substantially the same time.

2. The method of solidifying a casting comprising preheating a mold in which a casting is to be formed to a temperature above the freezing temperature of the material forming the casting, depositing into the mold liquid material to be solidified, evacuating the space within the mold above the liquid material therein, and uniformly cooling the body of liquid material in the mold throughout its cross section and bringing the temperature of the material throughout the body to its freezing temperature at substantially the same time.

3. The method of solidifying a casting comprising preheating a mold in which a casting is to be formed to a temperature above the freezing temperature of the material forming the casting, insulating the mold from external vibrations, depositing into the mold liquid mate rial to be solidified and uniformly cooling the body of liquid material in the mold throughout its cross section and bringing the temperature of the material throughout the body to its freezing temperature at substantially the same time.

4. The method of solidifying a casting comprising preheating a mold in which a casting is to be formed to a temperature above the freezing temperature of the material forming the casting, depositing into the mold liquid material to be solidified and cooling the body of liquid material in the mold and intermittently heating the outer portions of the body to prevent the outer portions of the body from cooling to its freezing temperature prior to the inner portion thereof.

5. The method of solidifying a casting comprising preheating a mold in which a casting is to be formed to a temperature above the freezing temperature of the material forming the casting, depositing into the mould liquid material to be solidified, insulating the mould from external vibrations, evacuating the space within the mould above the liquid material therein during cooling of the liquid, and cooling the body of liquid material in the cooling to its freezing temperature prior to the inner portion thereof. g V

6. The method of solidifying a casting comprising pouring a body of molten metal into a mold preheated to a temperature above the freezing temperature of the metal, cooling the body of molten metal substantially uniformly throughout its section to a temperature slightly below its freezing temperature and then inducing substantially simultaneous solidification of all parts of the body of metal.

7. The method of solidifying a hollow casting comprising pouring a body of molten metal into a rotatable mold preheated to a temperature above the freezing temperature of the metal, distributing the body of molten metal into annular form within the mold by rotating the mold, cooling the annular body of molten metal in the rotating mold substantially uniformly throughout its section to a temperature slightly below its freezing temperature andthen inducing substantially simultaneous solidification of all parts of the body of metal.

- 8. .Apparatus for substantially simultaneously solidify ing the entire body of a metal casting comprising a 1'0- tatable mold for receiving molten metal, means for sealing and evacuating the space above the metal in the mold,,means for heating the mold uniformly and metal therein while the mold is rotating, said heating means includes induction heating coils and a conductive material distributed over substantially the entire surface of .the mold and electrically heated upon energization of 'said induction heating coils, said conductive" material being positioned in contact with said mold and between said mold and said coils to provide for flow of heat therefrom to saidmold and to shield metal within the mold from the induction of electrical currents therein result- .ing in stirring of the metal, means for cooling the mold uniformly and metal therein while the mold is rotating, said cooling means being mounted in the mold and means for rotating the mold during heating and cooling thereof.

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